Diamond is a preferred material for semiconductor devices because it has semiconductor properties that are better than traditionally used silicon (Si), germanium (Ge) or gallium arsenide (GaAs). Diamond provides a higher energy band gap, a higher breakdown voltage and a higher saturation velocity than these traditional semiconductor materials.
These properties of diamond yield a substantial increase in projected cutoff frequency and maximum operating voltage compared to devices fabricated using Si, Ge, or GaAs. Si is typically not used at temperatures higher than about 200.degree. C. and GaAs is not typically used above 300.degree. C. These temperature limitations are caused, in part, because of the relatively small energy band gaps for Si (1.12 eV at ambient temperature) and GaAs (1.42 eV at ambient temperature). Diamond, in contrast, has a large band gap of 5.47 eV at ambient temperature, and is thermally stable up to about 1400.degree. C.
Diamond has the highest thermal conductivity of any solid at room temperature and exhibits good thermal conductivity over a wide temperature range. The high thermal conductivity of diamond may be advantageously used to remove waste heat from an integrated circuit, particularly as integration densities increase. In addition, diamond has a smaller neutron cross-section which reduces its degradation in radioactive environments, i.e., diamond is a "radiation-hard" material.
Because of the advantages of diamond as a material for semiconductor devices, there is at present an interest in the growth and use of diamond for high temperature and radiation-hardened electronic devices, particularly field-effect transistors which are fundamental building blocks of modern integrated circuits. For example, it is known in the art to form a diamond MESFET (metal-semiconductor field-effect transistor) using a rapid thermal processing (RTP) technique to drive in and activate boron in a type IIa diamond substrate. See Tsai et al. in Diamond MESFET Using Ultrashallow RTP Boron Doping, IEEE Electron Device Letters, Vol. 12, No. 4, pp. 157-159, April 1991. The MESFET has a metal-semiconductor rectifying contact as the gate electrode. The Tsai article proposes using cubic boron nitride as a thermal diffusion source to introduce boron as the p-type dopant using an RTP technique at high temperature (i.e., about 1400.degree. C.). Unfortunately, such high temperature processing is beyond the capability of most commercially available processing chambers which are typically only operable to about 1200.degree. C. Moreover, the type II diamond used in this MESFET includes nitrogen which produces nitrogen donors and compensates a portion of the acceptors. This compensation phenomena reduces the effectiveness of a portion of the boron dopant.
Another field-effect transistor is proposed by Hewett et al. in a paper entitled Fabrication of an Insulated Gate Diamond FET for High Temperature Applications, presented at the International High Temperature Electronics Conference in Albuquerque New Mexico in June of 1991. The IGFET (insulated gate field-effect transistor) includes a boron-doped diamond layer formed by uniformly doping the diamond layer with multiple ion implantation steps. Hewett suggests that a significant improvement in specific contact resistance may be obtained by more heavily doping the source and drain regions of the device, such as by using an additional ion implantation step for these regions. Unfortunately, the multiple implantation steps must be carried out at very low temperatures (i.e., about 77.degree. K.), and therefore require liquid nitrogen cooling.
Gildenblat, et al. in High-Temperature Thin-Film Diamond Field-Effect Transistor Fabricated Using a Selective Growth Method, IEEE Electron Device Letters, Vol. 12, No. 2, pp. 37-39, Feb. 1991, proposes a method for making a diamond MOSFET (metal-oxide semiconductor field-effect transistor) by depositing a homoepitaxial diamond film on a substrate. Boron doping is achieved in situ by placing boron powder on the substrate holder before deposition of the diamond layer. During diamond layer growth, the plasma etches the boron powder and forms boron hydrides. The boron is thus incorporated into the diamond layer in an expectedly uniform distribution. The MOSFET includes an insulating silicon dioxide layer separating the gate electrode from the active channel region of the diamond layer. Similarly, it has been proposed to use diborane as a gaseous source for plasma deposition in conjunction with the diamond layer formation. See Fountain et al. in IGFET Fabrication on Homoepitaxial Diamond Using In Situ Boron and Lithium Doping, presented at the Electrochemical Society Meeting held in Washington D.C. in May 1991.
Despite advancements in the art of diamond field-effect transistors, improvements are still necessary to produce commercially acceptable devices operable at temperatures beyond the limits of conventional semiconductor materials. Low resistance ohmic contacts for the source and drain electrodes of a diamond field-effect transistors are also desirable to improve operating performance. Moreover, the prior art processing steps for making the boron-doped diamond layers for field-effect transistors have required either complicated and expensive liquid nitrogen cooling, or have required high temperatures beyond the range of typical commercially available processing equipment. In addition, the prior art field-effect transistors have required expensive single crystal diamond as a component thereof.